There are two main types of semiconductor storage devices: RAM (random access memory) and ROM (read only memory). There is no limit on the number of write cycles for RAM, but because the stored data is deleted when the power is turned off, the problem with RAM is that the power consumption for storing data during standby becomes large. On the other hand, ROM can store data even after the power is turned off, but ROM has a limited number of write cycles, and thus, cannot be used where frequent rewriting is needed. Thus, research and development of NVRAM (non-volatile random-access memory) that has no limit in the number of writing cycles and that can store data written with very low power consumption has been in progress for a long time, but NVRAM has not yet been made into a product.
Insulated-gate FETs such as a MOSFET that use an oxide semiconductor with a wider bandgap than silicon are anticipated to greatly reduce the leakage current compared to silicon MOSFETs, and NVRAM developed using a MOSFET made of an oxide semiconductor is reported in Non-Patent Document 1 below.
As shown in FIG. 6, a memory cell disclosed in Non-Patent Document 1 is provided with a regular silicon MOSFET 30, a capacitor 31 that has an end connected to a gate FN of the silicon MOSFET 30, and an oxide semiconductor MOSFET 32 in which the gate FN is connected to a source or a drain of the MOSFET 32. By using the silicon MOSFET 30 and the capacitor 31, an equivalent to a silicon MOSFET (memory device) having a stacked gate structure in which a floating gate and a control gate used in conventional memory cells are layered is formed. In conventional flash memories, input and output of charges from the floating gate are performed via a thin gate insulating film by hot carrier injection, FN tunneling, or the like. However, in the case of a memory cell with a circuit configuration that is shown in FIG. 6, the input and output of charges is performed via the oxide semiconductor MOSFET 32. As a result, there is no need to form a strong electric field for inputting and outputting charges from the floating gate FN, and the data can be overwritten with low voltage and at a high speed; thus, there is no limit to the number of write cycles. Furthermore, because the leakage current of the oxide semiconductor MOSFET 32 is small, the stored charge of the floating gate FN can be stored stably for a long period of time.